There are many situations where it is desired to identify chemical compounds in a sample. Such samples may be taken directly from the environment or they may be provided by front end specialized devices to separate or prepare compounds before analysis. Furthermore, recent events have seen members of the general public exposed to dangerous chemicals in situations where previously no thought was given to such exposure. There exists, therefore, a demand for low cost, accurate, easy to deploy and use, reliable devices capable of identifying the chemical content of a sample.
As well, recent events have brought renewed interest to the addition of taggants to explosive materials for security purposes. Use of taggants serves two different functions and thus uses two different kinds of taggants. Detection taggants are materials added to explosives that can be sensed prior to detonation by appropriate detection equipment. Identification taggants are additives designed to survive an explosive blast, to be recoverable at the bomb scene, and to provide traceable sourcing information related to the explosives' purchase history.
Taggant options include addition of volatile chemicals, radioisotopes and the like. In countries participating under international commercial air security conventions, one of several volatile chemicals can be used to mark plastic explosives for detection. The United States has officially designated 2,3-dimethyl-2,3-dinitro-n-butane (C6H12N2O4), commonly referred to as DMNB, as a detection taggant for plastic explosives.
These taggants and other explosive compounds can be detected with various analytical equipment. One class of known chemical analysis instruments is referred to as mass spectrometers. Mass spectrometers are generally recognized as being one of the most accurate type of detectors for compound identification, given that they can generate a fingerprint pattern for even fragment ions. However, mass spectrometers are quite expensive and large and are relatively difficult to deploy in the field. Mass spectrometers also suffer from other shortcomings such as the need to operate at low pressures, resulting in complex support systems. These systems also require a highly trained user to tend to operations and interpret results.
Another class of known chemical analysis instruments enable use of atmospheric-pressure chemical ionization. Ion analysis is based on the recognition that ion species have different ion mobility characteristics under different electric field conditions at elevated pressure conditions including atmospheric pressure. Practices of the concept include time-of-flight Ion Mobility Spectrometry (IMS) and differential mobility spectrometry (DMS), the latter also sometimes referred to as field asymmetric waveform ion mobility spectrometry (FAIMS). These systems enable chemical species identification at atmospheric pressure, preferably based on dry and clean gas samples.
In a conventional time-of-flight IMS device (sometimes referred to as TOF-IMS), a propelling DC field gradient and a counter gas flow are set and an ionized sample is released into the field which flows to a collector electrode. Ion species are identified based on the DC field strength and time of flight of the ions to the collector. The ion mobility is constant when the electric field is weak.
DMS systems identify ion species by mobility behavior in a compensated high asymmetric RF field, where ions flow in a carrier gas and are shifted in their path by a high-low varying electric field. The conventional DMS operates with a selected Vrf and species detections are correlated with a preset, or scanned, DC compensation voltage (Vc) applied to the RF field. Species are identified based upon correlation of Vrf and Vc with historical detected data. The amount of compensation depends upon species characteristics and the selected compensated field conditions.
A typical DMS device includes a pair of opposed filter electrodes defining an analytical gap between them in a flow path (also known as a drift tube). Ions flow into the analytical gap. The asymmetric RF field (sometimes referred to as a filter field, a dispersion field or a separation field) is generated between the electrodes transverse to the carrier gas/ion flow in the gap. Field strength, E, varies as the applied RF voltage, Vrf (sometimes referred to as dispersion or separation voltage) and size of the gap between the electrodes. Such systems can operate at atmospheric pressure.
Ions are displaced transversely by the RF field, with a given species being displaced a characteristic amount toward the electrodes per cycle. The DC compensation voltage (Vc) is applied to the electrodes along with the Vrf to compensate the displacement of a particular species. Now the applied compensation will offset transverse displacement generated by the applied Vrf for that particular ion species. The result is zero or near zero net transverse displacement of that species, which enables that ion species to pass through the filter for detection. All other ions undergo a net displacement toward the filter electrodes and will eventually be neutralized upon contact with one of the filter electrodes.
If the compensation voltage is scanned for a given RF field, a complete spectrum of ion species in the sample can be produced. The recorded image of this spectral scan is sometimes referred to as a “mobility scan”, as an “ionogram”, or as “DMS spectra”. The time required to complete a scan is system dependent.
DMS operates based on the fact that an ion species will have an identifying property of high and low field mobility in the analytical RF field. Thus DMS detects differences in an ion's mobility between high and low field conditions and classifies the ions according to these differences. These differences reflect ion properties such as charge, size, and mass as well as the collision frequency and energy obtained by ions between collisions and therefore enables identification of ions by species.
Various chemical species in a sample can be identified according to the conventional DMS process. However, accurate identification of several species in a sample whose detection spectra overlap is difficult. This is in part due to the fact that DMS detection peaks are relatively broad compared to a mass spectrometer, so overlap is more likely than with a mass spectrometer. In fact, where several ion species exhibit similar behavior in the DMS filter field their associated DC compensation will be very close, and so their detection spectra (detection peaks) may overlap.
This “overlap” of detection peaks can interfere with species identification. But discrimination between overlapping spectra is not easily achieved and similar species may be difficult to separate from each other.
Furthermore, false negative detections are perilous when dangerous compounds, such as explosives, are at issue, whereas false positives can reduce trust in a detection system. Therefore improved spectrometer performance is an important goal.
It would therefore be desirable to provide a system and method for detecting ions with a differential ion mobility spectrometer (DMS, also known as a Field Asymmetric waveform Ion Mobility Spectrometer (FAIMS)), with enhanced sensitivity and discrimination between ion species. It would also be desirable to find optimal selectivity conditions by modifying transport gas modifiers and concentrations, in particular for detecting explosives by using a compact “micro-fabricated” DMS device.
Furthermore, while prior art IMS system scans take on the order of a second to complete, prior art DMS scans may take on the order of 10 seconds to complete. It would therefore be desirable to provide a DMS-based explosives detection system that enables rapid characterization of a chemical sample and accurate identification of explosives and capable of performing DMS scans in only a second or even a fraction of a second.